La Boca Formation Research Articles

Detrital zircon U-Pb ages of sandstones in continental red beds at Valle de Huizachal, Tamaulipas, NE Mexico: Record of Early-Middle Jurassic arc volcanism and transition to crustal extension

Continental strata and volcanic rocks of the type Huizachal Group in Valle de Huizachal record arc magmatism and subsequent crustal extension prior to seafl oor spreading in the Gulf of Mexico. The older La Boca Formation consists of two informal members, a lower unit of siliciclastic and volcanic rocks discordantly overlain by a predominantly siliciclastic upper member. The younger La Joya Formation is an upwardfi ning, alluvial-braided fl uvial succession with a basal conglomerate. U-Pb detrital-zircon ages (n = 576) from six Huizachal Group sandstones (fi ve from La Boca and one from La Joya) consist of four groups indicating a mixed provenance: (1) Grenville grains (~1.3‐1.0 Ga) derived from Gondwana (Novillo Gneiss); (2) earlymiddle Paleozoic grains (430‐300 Ma) derived from peri-Gondwanan accreted rocks (Granjeno Schist); (3) Permo-Triassic grains (296‐222 Ma) derived from volcanic and plutonic rocks (West Pangaean arc) and/or turbidites (Guacamaya Formation); and (4) Early-Middle Jurassic grains (199‐ 164 Ma), locally derived from the Nazas arc. Groups 1‐3 increase in abundance upsection as a result of unroofi ng of Jurassic volcanic and sedimentary carapace from uplifted basement. The Huizachal Group records three stages in the pre-breakup history of Gondwana: (1) The lower member of La Boca Formation (maximum depositional age 184‐183 Ma; Pleinsbachian) indicates Nazas arc activity; (2) the upper member (maximum depositional age 167‐163 Ma; Bathonian‐Callovian) indicates continued arc magmatism as early crustal extension formed horsts that supplied basement grains to an incipient rift basin; and (3) the La Joya Formation represents late rift basin development and widespread exposure of fl anking basement rocks. Although our La Joya sample lacks a coherent age group of young grains, its single youngest grain age (164 ± 3 Ma; Callovian) is consistent with its stratigraphic position beneath inferred Oxfordian strata.

Paleomagnetism of the Todos Santos and La Silla Formations, Chiapas: Implications for the opening of the Gulf of Mexico

We report paleomagnetic data for the Lower to Middle Jurassic La Silla and Todos Santos formations of southern Mexico, in west-central Chiapas and the Tehuantepec Isthmus region. Volcanic rocks and red beds of these formations were deposited prior to or during the early stages of Gulf of Mexico opening. Dual-polarity characteristic magnetizations reside primarily in hematite and pass intraformational conglomerate, regional tilt, and reversal tests; they are thus interpreted as primary magnetizations. Our sampling sites are concentrated in three localities; around La Angostura Lake, 17 accepted sites yield a tilt corrected mean of declination (Dec) = 325°, inclination (Inc) = 4.6° ( k = 11.9, α 95 = 10.8°); in the Matias Romero region, the mean is Dec = 312.9°, Inc = 3.2° (based on only seven sites); and in the Custepec area, Jurassic andesitic dikes intruding rocks of the Permian Chiapas Massif yield a corrected mean of Dec = 335.0°, Inc = 5.0° (six sites). The mean directions are discordant with respect to expected North America reference directions, and indicate a counterclockwise rotation of 35° to 40°. Inclinations indicate deposition or emplacement at near equatorial paleolatitudes (2.1°N ± 3.4°). This paleolatitude estimate is statistically indistinguishable from those previously observed in the La Boca Formation of the Huizachal Group in northeast Mexico. The localities we sampled in southern Mexico are separated by ∼150 km, suggesting that the paleomagnetic signature of these rocks reflects regional-scale rather than local deformation. These Jurassic paleomagnetic directions support a rotational origin for the Gulf of Mexico. The data are also consistent with an Early to Middle Jurassic reconstruction that places the Chiapas Massif offshore the Tamaulipas state in the western Gulf of Mexico, and an Euler rotation pole for the Maya Block for this time period in the eastern gulf. The apparent polar wander path defined by paleomagnetic poles for the Chiapas Massif and Jurassic rocks reported here suggests that relative motion between North America and the Maya Block occurred between Late Permian and Early Jurassic time, during a protracted rifting phase, and then in the Late Jurassic in association with seafloor formation in the Gulf.

Late Triassic stratigraphy and facies from northeastern Mexico: Tectonic setting and provenance

Triassic strata in northeastern Mexico distributed over an area of ∼120,000 km 2 have received relatively little attention in paleogeographic and tectonic reconstructions of western equatorial Pangea. Triassic marine sequences of the Zacatecas Formation in western San Luis Potosi and Zacatecas constitute facies corresponding to different sections of a submarine fan system previously described as the Potosi fan and were deposited on the paleo-Pacific margin of Pangea. New geochronologic data and field observations permit revision of the stratigraphy from Late Triassic to Early Jurassic fluvial deposits of the Huizachal Group that crop out in Nuevo Leon and Tamaulipas and indicate a link between the lower part of this succession (Triassic strata) and the Potosi fan. This work proposes and defines the El Alamar formation, which represents the only Triassic strata of the Huizachal Group (lower part of the Late Triassic–Early Jurassic La Boca Formation), and is interpreted as a continental succession that records a major fluvial system draining equatorial Pangea and flowing west into the Potosi fan. Petrographic and geochemical studies indicate that both Triassic successions, the Zacatecas Formation (marine) and El Alamar formation (continental) have continental block and recycled orogenic provenances. U-Pb detrital zircon geochronology by laser ablation–multicollector–inductively coupled plasma–mass spectrometry show three main zircon age populations: (1) Grenvillian (1300–900 Ma); (2) Pan-African (700–500 Ma); and (3) dominant Permian–Triassic (280–240 Ma). The presence of these zircons displays evidence of sources in Grenvillian Oaxaquia block, Pan-African terrains such as Yucatan and southeastern Texas, and a prominent contribution from the Permian–Triassic east Mexico magmatic arc. Notable is the absence of detrital contributions from the southwestern North American craton. The geochronological data thus argue against proposed southeastward displacement of Triassic successions and their basement to their current position as proposed by the Mojave-Sonora megashear hypothesis. We propose that continental Triassic strata were deposited in eastern Mexico before the opening of the Gulf of Mexico basin, and are thus autochthonous and transported detritus toward the ancient Pacific margin into the Potosi fan, which is also autochthonous.

Late Early Jurassic mammaliaforms from Huizachal Canyon, Tamaulipas, México

ABSTRACT Previous reports of the fossil vertebrates from the upper Lower Jurassic La Boca Formation of Tamaulipas, México, presented descriptions of a tritylodontid therapsid, a burrowing diapsid, crocodyliforms, a pterosaur and sphenodontians. Mammaliaform material is rare, consisting primarily of partial lower jaws, some of which are extremely small. Five partial dentaries and one maxilla all have molars of “triconodont” morphology. Two lower jaws clearly show the internal mandibular trough, suggesting attachment of postdentary bones, another two do not possess a trough, and in another its presence or absence cannot be determined. This diversity of forms suggests that during the late Early Jurassic variations on the primitive “triconodont” molar patterns and in lower jaw morphology took place. The mammaliaform specimens from Tamaulipas appear to represent new taxa, but, because of the incomplete preservation of most of the material, only three new genera and species are erected: Bocaconodon tamaulipensis, Victoriaconodon inaequalis, and Huasteconodon wiblei.

Lower Miocene Stratigraphy along the Panama Canal and Its Bearing on the Central American Peninsula

Before the formation of the Central American Isthmus, there was a Central American Peninsula. Here we show that southern Central America existed as a peninsula as early as 19 Ma, based on new lithostratigraphic, biostratigraphic and strontium chemostratigraphic analyses of the formations exposed along the Gaillard Cut of the Panama Canal. Land mammals found in the Miocene Cucaracha Formation have similar body sizes to conspecific taxa in North America, indicating that there existed a terrestrial connection with North America that allowed gene flow between populations during this time. How long did this peninsula last? The answer hinges on the outcome of a stratigraphic dispute: To wit, is the terrestrial Cucaracha Formation older or younger than the marine La Boca Formation? Previous stratigraphic studies of the Panama Canal Basin have suggested that the Cucaracha Formation lies stratigraphically between the shallow-marine Culebra Formation and the shallow-to-upper-bathyal La Boca Formation, the latter containing the Emperador Limestone. If the La Boca Formation is younger than the Cucaracha Formation, as many think, then the peninsula was short-lived (1–2 m.y.), having been submerged in part by the transgression represented by the overlying La Boca Formation. On the other hand, our data support the view that the La Boca Formation is older than the Cucaracha Formation. Strontium dating shows that the La Boca Formation is older (23.07 to 20.62 Ma) than both the Culebra (19.83–19.12 Ma) and Cucaracha (Hemingfordian to Barstovian North American Land Mammal Ages; 19–14 Ma) formations. The Emperador Limestone is also older (21.24–20.99 Ma) than the Culebra and Cucaracha formations. What has been called the “La Boca Formation” (with the Emperador Limestone), is re-interpreted here as being the lower part of the Culebra Formation. Our new data sets demonstrate that the main axis of the volcanic arc in southern Central America more than likely existed as a peninsula connected to northern Central America and North America for much of the Miocene, which has profound implications for our understanding of the tectonic, climatic, oceanographic and biogeographic history related to the formation of the Isthmus of Panama.

Diagenesis in porosity evolution of opening-mode fractures, Middle Triassic to Lower Jurassic La Boca Formation, NE Mexico

Opening-mode fractures (joints) in Middle Triassic to Lower Jurassic La Boca Formation sandstones, northeastern Mexico, have patterns of fracture porosity, mineral-fill structures, and size distributions not previously described from outcrop. Patterns match those found in cores from many basins. We used aperture measurements along lines of observation (scanlines), fracture-trace maps, petrography, high-resolution scanning-electron-microscope-(SEM)-based cathodoluminescence, and fluid inclusions to characterize fracture populations. Open fractures are lined by quartz that precipitated while fractures were opening, whereas sealed fractures additionally contain calcite deposited after fractures ceased opening. Large fractures and arrays of contemporaneous microfractures have consistent power-law aperture-size scaling over approximately three orders of magnitude. Our results imply that open fractures and fracture sizes depend on diagenetic state. The interplay of fracture mechanics and diagenetic history is a determinant on effective porosity within fractures and, thus, open fracture persistence, connectivity, and fluid flow.

A dwarf sphenodontian from the Jurassic La Boca Formation of Tamaulipas, México

ABSTRACT A new genus and species of sphenodontian lepidosaur, Zapatadon ejidoensis, is described, based upon a nearly complete skull of a posthatchling specimen from the late Early Jurassic La Boca Formation, Tamaulipas, México. It is diagnosed by its extremely small size, the presence of a depression around the hatchling tooth series on the dentary, an expanded prefrontal that surrounds most of the dorsal process of the maxilla, and a broad jugal that nearly excludes the maxilla from the orbit. A phylogenetic analysis of its relationships supports the hypothesis that it is among the closest relatives of the only extant sphenodontian, Sphenodon. However, the relationships of Z. ejidoensis could not be resolved with respect to Eilenodontinae, Opisthias, and a Cynosphenodon-Sphenodon clade. The most significant character diagnosing the clade composed of these five taxa is wear facets indicative of propalinal mandibular movement.

Depositional environments of a Middle Jurassic terrestrial vertebrate assemblage, Huizachal Canyon, Mexico

ABSTRACT A large assemblage of vertebrate fossils from the La Boca Formation in Huizachal Canyon, near Ciudad Victoria, Tamaulipas, Mexico, is preserved in a unique setting: debris-flow sediments deposited over a landscape with at least modest topographic relief. Angular feldspar lathes in the rock suggest that deposition was associated with penecontemporaneous volcanism. The ~8,000-specimen collection from Huizachal Canyon includes the tritylodontid Bocatherium mexicanum, the burrowing diapsid Tamaulipasaurus morenoi, three types of mammals, two crocodyliforms, two types of dinosaurs, a pterosaur, and three sphenodontians. The fossils generally occur as small (~5 mm), isolated bones or bone fragments, but 35 articulated or associated specimens have been found. No turtles or fishes have been identified, implying a terrestrial source for this assemblage. Biostratigraphic and preliminary radiometric evidence suggest an early Middle Jurassic age, making this assemblage the only substantial terrestrial Middle...

A new burrowing diapsid from the Jurassic La Boca Formation of Tamaulipas, Mexico

ABSTRACT Two articulated skulls and associated material from the Early or Middle Jurassic La Boca Formation of Mexico represent a new genus and species of small diapsid, Tamaulipasaurus morenoi. The skull is superficially similar to that of burrowing squamates, especially amphisbaenians and dibamids, but the presence of plesiomorphic characters, such as a complete lower temporal bar, contradict a phylogenetic relationship within Squamata. Among its autapomorphies are extensive fusion of braincase bones and of postdentary bones, a broad thin squamosal, absence of three circumorbital bones, an extensive side wall to the braincase, an anteriorly directed fenestra ovalis, an unusual course for the stylar process of the stapes in a groove between the quadrate and braincase, and a large longitudinal foramen through the base of the transverse process of cervical vertebrae. The phylogenetic relationships of Tamaulipasaurus morenoi are difficult to assess because its burrowing adaptations obscure its structure. It...

Studies in Neotropical Paleobotany. III. The Tertiary Communities of Panama-Geology of the Pollen-Bearing Deposits

Samples containing fossil pollen and spores were collected from six sites representing five Tertiary formations in Panama. The Gatuncillo Formation is middle(?) to late Eocene, based on foraminifera and mollusk fossils. The formation is exposed around Madden Basin and samples were collected in the vicinity of Alcalde Diaz (Pefioncito). Material from the early Miocene Culebra Formation was obtained from a well drilled in the Contractor's Hill locality at the base of Gold Hill. The Cucaracha Formation overlies the Culebra and is also early Miocene in age. The Cucaracha material was collected from exposures along the Gaillard Cut section of the Canal. The early Miocene La Boca Formation outcrops in the same area. Although these three formations are similar in age (early Miocene), field evidence demonstrates that they are sequential from the Culebra (oldest), through the Cucaracha, to the La Boca (youngest). The Gatun Formation is late Miocene and early Pliocene in age and core samples were taken from a series of wells drilled in Gatun Lake. The location, stratigraphy, lithology, and depositional environments are summarized for these five formations. In subsequent publications the paleocommunities, paleoenvironments, and biogeographic implications will be considered for each of the microfloras. The present studies on the Cenozoic history of neotropical vegetation began in 1964 with a collection of pollen-bearing samples from Tertiary deposits in central Panama. Subsequently, other samples were obtained from the Oligocene of Puerto Rico and the Miocene of Veracruz, Mexico. At that time the vegetation of Central America was poorly known, but research by staff of the Missouri Botanical Garden, the Field Museum, and various institutions in Central America was rapidly providing new information on the composition of the flora, and on the range, ecology, and biogeographic relationships of its components. By comparison, the vegetation of more restricted areas to the north, such as Puerto Rico and Veracruz, was somewhat better known (Howard, 1973; Gomez-Pompa, 1973), and it seemed logical to begin paleofloristic studies on geologically younger deposits in these areas where modem analogs were more clearly defined. Consequently, our first studies were on the middle Oligocene San Sebastian palynoflora from Puerto Rico (Graham & Jarzen, 1969), followed by the late Miocene Paraje Solo palynoflora from Veracruz, Mexico (Graham, 1976). The decision to defer consideration of the material was fortunate for several reasons. The older Gatuncillo assemblage from proved the most difficult of our neotropical studies, and recently accumulated information on the modem vegetation was most helpful in making paleoenvironmental reconstructions. The completion of the Flora of Panama (1980) and Croat's (1978) Flora of Barro Colorado Island, initiation of Burger's Flora Costaricensis, and the Flora Mesoamerica, coordinated through the Missouri Botanical Garden and Kew, are making both specimens and information available to our studies on vegetational history. Recent biogeographic summaries are also valuable for interpreting the paleofloristic data (e.g., Gentry, 1982). Also, the pollen and spore reference collection, used for identification of the fossil palynomorphs, has increased from 10,000 slides in 1964 to nearly 22,000. From the standpoint of geology, the recently revised Geologic Map of the Canal and Vicinity, Republic of Panama (Stewart et al., 1 980) and modem plate tectonic summaries (e.g., Coney, 1982; Raven & Axelrod, 1974) are important for placing the paleobotanical data in a sound geologic context. Pollen-bearing samples from five Tertiary formations were collected in within or adjacent to the former Canal Zone (Figs. 1-3). These I Supported by NSF grants DEB-8205926, DEB-8007312, and GB-1 1862. 2 Department of Biological Sciences, Kent State University, Kent, Ohio 44242. 3Chief Geologists (retired), Canal Commission. Present address: 204 Tampa Downs Blvd., Lutz, Florida 33549. ANNm. MISSOURI BOT. GARD. 72: 485-503. 1985. This content downloaded from 157.55.39.104 on Mon, 20 Jun 2016 06:29:04 UTC All use subject to http://about.jstor.org/terms 486 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 72

A paleomagnetic study of certain mesozoic formations in Northern Mexico — Comments

Abstract Rocks of Late Cretaceous, Early Jurassic and Late Triassic age, collected in northern Mexico yield the following pole positions: 169.3°E57.9°N (Cretaceous), 70.7°E76.0°N (?Jurassic) and 119.2°E76.4°N (?Late Triassic). The Triassic and Cretaceous poles are not significantly different from those class-A poles (Hicken et al., 1972) of the North American craton. It is therefore suggested that the North American craton may be traced south as far as 23°N and inferentially a further four degrees (to the Mexican volcanic belt). The results from the La Boca Formation are interpreted as indicating a much greater age (Late Precambrian-Early (Paleozoic) than is currently assigned to that formation.

A paleomagnetic study of certain Mesozoic formations in northern Mexico

Abstract Rocks of Late Cretaceous, Early Jurassic and Late Triassic age, collected in northern Mexico yield the following pole positions: 169.3°E57.9°N (Cretaceous), 70.7°E76.0°N (?Jurassic) and 119.2°E76.4°N (?Late Triassic). The Triassic and Cretaceous poles are not significantly different from those class-A poles (Hicken et al., 1972) of the North American craton. It is therefore suggested that the North American craton may be traced south as far as 23°N and inferentially a further four degrees (to the Mexican volcanic belt). The results from the La Boca Formation are interpreted as indicating a much greater age (Late Precambrian-Early (Paleozoic) than is currently assigned to that formation.

Age and Correlation of Huizachal Group (Mesozoic), State of Tamaulipas, Mexico

Field studies in the Huizachal anticlinorium, Sierra Madre Oriental, near Ciudad Victoria, Tamaulipas, Mexico, disclose that the Huizachal group (redbeds), formerly undivided, contains at least two mappable sequences, herein named La Joya formation and La Boca formation. The La Boca formation consists mostly of red and green claystones, mudstones, siltstones, sandstones, and conglomerates and rests unconformably on (1) crystalline rocks of unknown age and (2) deformed Paleozoic strata lithically similar to those of the Marathon uplift (Texas). It is considerably folded and faulted and underlies either the La Joya formation (redbeds) or the overlying Zuloaga limestone (Late Jurassic) with angularity up to almost 90°. The La Joya includes (1) a lithically variable basal conglomerate, (2) a thin limestone and limestone conglomerate, (3) red mudstones, claystones, siltstones, and fine quartzose sandstones which grade upward into red, pink, or green, fine to very coarse, cross-bedded, quartzose sandstones and red, pink, or green conglomerates. The La Joya possesses conformable to low angular unconformable relations with the superjacent Zuloaga limestone; it rests with considerable angularity on the La Boca formation and on deformed sediments of late Paleozoic age. At some places it overlies crystalline rocks probably Paleozoic and Precambrian in age. Fossil plants from the upper part of the La Boca formation in Novillo Canyon indicate that this part of the group is probably late Triassic in age. The La Joya thus appears to be Jurassic in age, its general relations to the Zuloaga limestone (of definite Late Jurassic age) being such as to suggest that it might be Middle or Late Jurassic in age.

Tertiary Formations of Panama Canal Zone and Adjoining Parts of Panama

The Tertiary formations of the Canal Zone and adjoining parts of Panama range in age from late Eocene to late Miocene, or possibly early Pliocene. New formation and member names--Gatuncillo formation, Quebrancha limestone member of Caimito formation, La Boca formation, and Pedro Miguel agglomerate--proposed in official Panama Canal reports are defined. Fossils from Vamos Vamos, a submerged locality in the Gatun Lake area, are considered early Oligocene in age. The fossiliferous strata at that locality are inferred to represent a marine tongue in the essentially non-marine Bohio formation. The late Eocene Gatuncillo formation is strongly transgressive; the middle and late(?) Miocene Gatun formation is also transgressive; the late Oligocene(?) and early Miocene Culebra formation and the late Miocene or early Pliocene(?) Chagres sandstone represent minor overlaps.